Sensitized photochemical switching for cholesteric liquid crystal displays

ABSTRACT

The present invention relates to photo-tunable dopant compositions comprising a photo-reactive chiral compound capable of undergoing a photochemical reaction resulting in the loss of chirality, and a triplet sensitizer. The present invention also relates to a display comprising a substrate, a liquid crystalline layer thereon, wherein the liquid crystalline layer comprises a nematic host, at least one chiral dopant, a photo-reacted compound, and a triplet sensitizer, and at least one transparent conductive layer. The present invention also relates to a method of tuning a cholesteric liquid crystal material comprising providing at least one mesogenic compound, at least one triplet sensitizer, and at least one photo-reactive chiral compound; combining the at least one mesogenic compound, at least one triplet sensitizer, and at least one photo-reactive chiral compound to form a mixture; and irradiating the mixture for a period of time.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a divisional of U.S. patent application Ser. No.11/403,970, filed on Apr. 13, 2006, entitled “SENSITIZED PHOTOCHEMICALSWITCHING FOR CHOLESTERIC LIQUID CRYSTAL DISPLAYS”. This application isincorporated herein by reference in its entirety.

FIELD OF THE INVENTION

The present invention relates to photo-tunable chiral dopants (PTCDs)for use in applications as memory elements, display, or coatings forwave guides

BACKGROUND OF THE INVENTION

Chiral nematic (also referred to as cholesteric) liquid crystals can bemade to exhibit different optical states under different electricalfield conditions and are characterized by a unique combination ofproperties, including optical bistability, making them particularlyuseful in display applications. Depending upon the magnitude and shapeof the electric field, the optical state of the material can be changedbetween light scattering and light reflecting states, or any one of anumber of intermediate states there between which can be made to reflectany desired intensity of colored light along a continuum of such states,thus providing gray scale.

With such chiral nematic materials, a low electric field pulse generallyresults in a light scattering focal conic texture. The application of asufficiently high electric field pulse, i.e., an electric field highenough to homeotropically align the liquid crystal directors, will, uponremoval of the pulse, drive the material to a light reflecting twistedplanar texture that can be any desired color depending upon the pitchlength of the liquid crystal. The light scattering and light reflectingstates can be made to remain stable at zero field. If a sufficientlyhigh electric field is maintained, the material is transparent until thefield is removed. When the field is turned off quickly, the materialreforms to the light reflecting state and, when the field is turned offslowly, the material reforms to the light scattering state. Electricfield pulses of various magnitudes below that necessary to drive thematerial from the stable reflecting state to the stable scattering statewill drive the material to intermediate states that are themselvestypically stable. These multiple stable states indefinitely reflectcolored light of an intensity between that reflected by the reflectingand scattering states. Thus, depending upon the magnitude of theelectric field pulse the material exhibits stable gray scalereflectivity. Application of mechanical stress to the material can alsobe used to drive the material from the light scattering to the lightreflecting state.

The color reflected by a chiral nematic liquid crystal depends upon thepitch length of the liquid crystal, which is in turn dependent upon theamount of chiral material in the liquid crystal. The pitch length of theliquid crystal materials may be adjusted based upon the followingequation (1):λ_(max)=n_(av)p_(o)  (1)where λ_(max) is the peak reflection wavelength, that is, the wavelengthat which reflectance is a maximum, n_(av) is the average index ofrefraction of the liquid crystal material, and p_(o) is the naturalpitch length of the chiral nematic helix. Definitions of chiral nematichelix and pitch length and methods of its measurement, are known tothose skilled in the art such as can be found in the book, Blinov, L.M., Electro-optical and Magneto-Optical Properties of Liquid Crystals,John Wiley & Sons Ltd. 1983.

The wavelength of the reflected light can also be controlled byadjusting the chemical composition, since cholesterics can eitherconsist of exclusively chiral molecules or of nematic molecules with achiral dopant dispersed throughout. In this case, the dopantconcentration is used to adjust the chirality and thus the pitch. Formost concentrations of chiral dopants, the pitch length induced by thedopant is inversely proportional to the concentration of the dopant. Theproportionality constant is given by the following equation (2):p _(o)=1/(HTP.c)  (2)

where c is the concentration of the chiral dopant and HTP (helicaltwisting power) is the proportionality constant.

One of the original approaches to photo-tuning of helical pitch lengthand λmax, of cholesteric liquid crystals was to add chiral dopants thatwere photoactive (see, A. Yu. Bobrovsky, N. I. Boiko and V. P. ShibaevLiq. Cryst. 25 (1998), p. 679.; A. Yu. Bobrovsky, N. I. Boiko and V. P.Shibaev Liq. Cryst. 26 (1999), p. 1749; P. van de Witte, J. C. Galan andJ. Lub Liq. Cryst. 24 (1998), p. 819; M. Brehmer, L. Lub and P. van deWitte Adv. Mater. 10 (1998), p. 1438).

The main principle of the development of such light-controllable liquidcrystal is based on the synthesis of photochromic copolymers whosemacromolecules consist of mesogenic (as a rule, nematogenic) andcombined photo-tunable chiral dopant (PTCD) groups, which are chemicallylinked in the common monomer unit. In this case, mesogenic fragments areresponsible for the formation of the nematic phase, chiral groupsprovide the twisting of the nematic phase and formation of helicalsupramolecular structure. Finally, photo-tunable chiral dopant fragmentscan easily change their molecular structure under the light irradiation.

The irradiation with a certain wavelength leads to the photo-inducedtransformations of the photo-tunable chiral dopant affecting both theconfiguration and shape of the side-chain group. This leads to adecrease both in the anisotropy of photo-tunable chiral dopant group andthe helical twisting power of a given chiral group. A decrease inhelical twisting power leads to the untwisting of the cholesteric helix,which is accompanied by a shift in the selective light reflectionmaximum to longer wavelengths. Thus, using light irradiation as theexternal control factor, one may effectively modify the opticalproperties of polymer films by changing the local supramolecular helicalstructure.

As noted above, selective adjustment of the pitch length of a liquidcrystal, and hence the color reflected thereby, can be accomplished byusing photo-tunable chiral dopants (PTCDs). Irradiation of aphoto-tunable chiral dopant with, for example, ultra violet (UV) lightor other high energy source such as laser, results in conversion ofchiral photo-tunable chiral dopant to an achiral molecule or to aracemic mixture. When one or more photo-tunable chiral dopants areincluded in a chiral nematic liquid crystal material, the pitch lengthof the resulting liquid crystal mixture can be either extended orshortened by varying exposures to UV light. By irradiating differentregions of the material with different amounts of UV through the use ofmasking techniques, the pitch lengths of each region can be tuned toreflect a different color, thereby creating different colored pixels orregions of spot color in the liquid crystal material itself.

U.K. Patent Application No. GB 2355720 discloses a process of preparinga reflective film by using a photodegradable chiral compound. GB 2355720describes a process for preparing a reflective film by coating apolymerizable cholesteric liquid crystal (CLC) material onto asubstrate, aligning the material into planar orientation, andpolymerizing the material by exposure to actinic radiation,characterized in that the polymerizable material comprises at least onephotodegradable chiral compound that loses its chirality when beingexposed to actinic radiation. Also disclosed is the use of saidreflective film in optical, electrooptical, information storage,decorative and security applications, a liquid crystal display device,and a photodegradable compound.

U.S. Pat. No. 5,668,614 discloses a tunable chiral material (TCM) thatcan be changed from chiral to achiral or to a racemic mixture byirradiating with, for example, UV light or a high energy source such aslaser. Further disclosed is a light modulating liquid crystal cellcomprising a light modulating chiral nematic liquid crystal materialincluding a tunable chiral material, wherein different regions of theliquid crystal material exhibit different reflection wavelengths. Thecell is prepared by partially exposing the liquid crystal material withthe tunable chiral material to photo-irradiation, e.g. through aphotomask, leading to a change of the chirality of the tunable chiralmaterial and thus to a change of the helical pitch in the exposed partsof the chiral nematic liquid crystal material.

WO 98/57223 discloses a multi domain liquid crystal display devicecomprising a layer of nematically ordered liquid crystalline materialcontaining a chiral dopant sandwiched between two substrates. The liquidcrystal layer comprises at least two types of sub-pixels in which thetwist senses of the liquid crystalline material are mutually opposite,and the composition of the chiral dopant in the different types ofsub-pixels is different. The device is manufactured by sandwichingbetween the substrates a layer of liquid crystalline material containingan isomerisable chiral dopant with a first twist sense and anon-isomerisable chiral dopant with an opposite twist sense, andphotoirradiating the layer through a photomask. This causes theisomerisable dopant in the exposed parts of the layer to convert itschirality and thus its twist sense, leading to a change of the helicalpitch in the exposed parts.

However, in order to achieve desired change in chiral materials asdescribed in U.S. Pat. No. 5,668,614 and WO 98/57223, irradiation withUV light of high intensity and long duration is required. Therefore highlamp powers and long irradiation times are needed, which is a seriousdrawback for mass production. This is especially disadvantageous in casethe isomerizable chiral compound is used in a photocurable orphotopolymerizable liquid crystal mixture, where the light used toinduce a change in the isomerizable chiral compound also has theundesirable effect of inducing a premature polymerization process in themixture. Furthermore, UV irradiation of the mixture with high radiationdoses (i.e. high radiation intensities and long radiation periods) cancause undesired degradation of the other components of the liquidcrystal mixture.

PROBLEM TO BE SOLVED

There remains a need for liquid crystal materials, which are easy tomanufacture, stable to manufacturing exposure to light, and readilyavailable.

SUMMARY OF THE INVENTION

The present invention relates to a photo-tunable dopant compositioncomprising a photo-reactive chiral compound, wherein the photo-reactivechiral compound is capable of undergoing a photochemical reactionresulting in the loss of the chirality of the photo-reactive chiralcompound, and a triplet sensitizer. The present invention also relatesto a display comprising a substrate, a liquid crystalline layer thereon,wherein the liquid crystalline layer comprises a nematic host, at leastone chiral dopant, a photo-reacted compound, and a triplet sensitizer,and at least one transparent conductive layer. The present inventionalso relates to a method of tuning a cholesteric liquid crystal materialcomprising providing at least one mesogenic compound, at least onetriplet sensitizer, and at least one photo-reactive chiral compound,wherein the photo-reactive chiral compound is capable of undergoing aphotochemical reaction resulting in the loss of the chirality of thephoto-reactive chiral compound; combining the at least one mesogeniccompound, at least one triplet sensitizer, and at least onephoto-reactive chiral compound to form a mixture; and irradiating themixture for a period of time.

ADVANTAGEOUS EFFECT OF THE INVENTION

The present invention includes several advantages, not all of which areincorporated in a single embodiment. The present invention relies ontriplet sensitization rather than direct irradiation of a photo-tunablechiral dopant (PTCD). The inventive use of triplet sensitizers resultsin increased sensitivity to the photoradiation, and lower radiationdoses can be applied and degradation of cholesteric liquid crystalmaterial is prevented. Patterned films with different colors can beproduced using a cholesteric liquid crystal mixture containing thetriplet sensitizer and one or more photo-tunable chiral dopants,depending on the intensity and/or duration of radiation. The materialsof the invention are simple, stable and easily available molecules,which can be conveniently included into nematic liquid crystalmaterials. The optical changes brought about by the inventive processare large and can easily be detected. The invention is especially suitedto single layer patterning in a sheet comprising dispersed liquidcrystal domains such as a coating of polymer dispersed liquid crystals(PDLC) on a flexible plastic substrate.

DETAILED DESCRIPTION OF THE INVENTION

The present invention relates to a photo-tunable dopant compositioncomprising a photo-reactive chiral compound and a triplet sensitizer.The present invention utilizes the photo-tunable dopant in a cholestericliquid crystal material comprising a triplet sensitizer and one or morephoto-tunable chiral dopants (PTCD) that change chirality uponirradiation of the triplet sensitizer. The present invention alsoprovides methods of manufacturing.

The inventive use of triplet sensitizers results in increasedsensitivity to the photoradiation, compared to the direct irradiation ofphotoisomerizable chiral materials of prior art, therefore lowerradiation doses can be applied and degradation of cholesteric liquidcrystal material is prevented. Furthermore, the amount of degradation ofthe photo-tunable chiral dopant compound is proportional to amount oflight absorbed by the triplet sensitizer, so patterned films withdifferent colors can be produced using a cholesteric liquid crystalmixture containing the triplet sensitizer and one or more photo-tunablechiral dopant, depending on the intensity and/or duration of radiation.Because the invention relies on triplet sensitization rather than directirradiation of a photo-tunable chiral dopant, the irradiation can bedone at wavelengths longer than 350 nm.

The invention involves a photo-irradiation process that is efficient inthe use of light. As the reaction involves irradiation of a tripletsensitizer and then a triplet excited state energy transfer to aphoto-tunable chiral dopant, rather than direct irradiation of thephoto-tunable chiral dopant, quantum efficiencies are usually higher.This is mainly because direct light absorption by photo-tunable chiraldopants at wavelengths below 350 nm is very inefficient as most liquidcrystal materials have significant absorption below 350 nm. The degreeof conversion of photo-tunable chiral dopants is also higher with thetriplet sensitization process of this invention than with directexcitation of photo-tunable chiral dopants. A relatively large change inoptical properties can be obtained at low exposure to the irradiationlight.

The present invention addresses the problems in the prior art by using amixture comprising a triplet sensitizer and one or more photodegradablechiral compounds that irreversibly degrade upon photoirradiation of thetriplet sensitizer. The present invention involves the use of a tripletsensitizer and a photo-tunable chiral dopant for a liquid crystalmaterial in conjunction with photolithography or other maskingtechniques. The present invention uses a triplet sensitization schemefor efficient photoreactions of photo-tunable chiral dopants to addressissues regarding inefficient irradiation below 400 nm. In the inventivescheme, a triplet sensitizer molecule of appropriate triplet energy isirradiated to bring about the desired reaction in photo-tunable chiraldopant and induce change in the pitch length of chiral nematic liquidcrystal material. Thus, excitation of an appropriate triplet sensitizermolecule results in triplet energy transfer from the sensitizer to thephoto-tunable chiral dopant, which subsequently undergoes desiredreaction to reduce the chirality and change the pitch length. Sincedirect irradiation of photo-tunable chiral dopants is not required, thisscheme allows for selection of an appropriate triplet sensitizer thatcan be irradiated at longer wavelengths where photo-tunable chiraldopant doesn't have to have any absorption. An additional advantage ofthe present invention is increased quantum efficiency of the overallprocess.

Any compound whose chirality can be altered upon triplet sensitizationeither resulting in racemization or decomposition and which does notreact adversely with the other components of the system is contemplatedas being suitable for use in the invention. In other words, thepreferred compound is a photo-reactive chiral compound capable ofundergoing a photochemical reaction resulting in the loss of thechirality of photo-reactive chiral compound.

Although there are numerous photochemical mechanisms by which chiralityof various compounds may be destroyed or altered, the preferredmechanisms for use in the invention generally fall into two categories:

-   -   1. Triplet sensitized racemization and/or isomerization of the        photo-tunable chiral dopant;    -   2. Triplet sensitized bond cleavage reactions of the        photo-tunable chiral dopant.

The structure of a given photo-tunable chiral dopant compound willdepend upon its triplet energy and nature of its triplet sensitizedreaction. Since the photochemical racemization or isomerization and bondcleavage reactions and detailed mechanisms of these processes are wellknown to the skilled chemist, it would be well within the skill in theart to select and synthesize suitable compounds for use in the presentinvention once it is decided which triplet sensitized photochemicalprocess of photo-tunable chiral dopant will be employed.

As noted above, in triplet sensitized photochemical reactions there arebasically two ways to destroy the chirality of a photo-tunable chiraldopant bond cleavage process and racemization and/or isomerizationprocess. In triplet sensitized racemization process, energy transfer tothe photo-tunable chiral dopant molecule causes no structural change,other than conversion of one enantiomer into its mirror image(photo-racemization of an optically pure reagent). In this system, theultimate obtainable optical purity of the photo-tunable chiral dopant atthe photostationary state (γPSS) by irradiation with circularlypolarized light (CPL) is determined by Kuhn anisotropy. There are anumber of photo-tunable chiral dopants systems available that undergotriplet sensitized racemization and are capable of providing a γPSSlarge enough to be useful for switching a liquid crystal (see Table 2).

In the triplet sensitized bond cleavage reactions, the ultimate resultis elimination of a group or substituent from the chiral carbon, therebyrendering the carbon achiral, or to cleave the chiral carbon itself fromthe molecule, which destroys the chirality of both the original moleculeand the leaving group. Both of these methods find their basis insynthetic organic chemistry, where photoremovable protecting groups arefrequently used during synthesis. The essence of both is to select aknown photoremovable protecting group as is known in the art, employ itin a position where the removal of the protecting group destroys thechirality of the compound, and then use an appropriate tripletsensitizer to initiate the desired reaction. Photoremovable protectinggroups are discussed in some detail in Binkley, R. W., et al., SyntheticOrganic Chemistry, Plenum Press, New York, N.Y. (1984) (Chapter 7, pp.375-423), incorporated herein by reference. Known photoremovableprotecting groups include esters of sulfonic acids, carboxylic acids andcarboxylic acid esters, hydrazones (e.g., N,N-dimethylhydrazones),acetal forming dithio groups, 1,2-diphenylmaleimides, o-nitrophenylaminogroups, aryl azido ethers, benzoin esters (e.g., methoxy substitutedbenzoin esters), polymer bound phenacyl groups, phenacyl groups (e.g.,methoxyphenacyl), and benzyloxycarbonyl compounds. When incorporatedinto the structure of a chiral compound, such groups can be used todestroy the chirality of the compound upon triplet-sensitized reaction.The selection of appropriate groups for use in a photo-tunable chiraldopant will depend upon the compatibility of the particularphoto-tunable chiral dopant and its triplet sensitized reaction productswith the other components in the system.

A preferred category of photo-tunable chiral dopants includes aphotocleavable leaving group directly attached to the chiral carbon.Upon triplet-sensitized irradiation, the bond between the chiral carbonand the leaving group is cleaved, thereby destroying the chirality ofthe photo-tunable chiral dopant compound. There are numerousphotochemical mechanisms by which a labile substituent on a chiralcarbon may be eliminated by photoirradiation. Accordingly, there arecountless compounds falling within this category. However, as notedabove, the chemical mechanisms are well known and those of ordinaryskill in the art will know the necessary structural requirements neededfor a compound to undergo triplet sensitized bond cleavage withdestruction of chirality according to a given mechanism. Preferredcompounds of this category are typically characterized by an aromaticring or ring system having the chiral carbon bound to it directly, betato the ring through a carbon, a substituted carbon or a heteroatom suchas oxygen or sulfur. This is because the aromatic ring or ring systemacts to increase the lability of the leaving group. To enhance thisfunction of the ring or ring system it may often be activated by variouselectron-withdrawing substituents such as one or more nitro groups as isknown in the art. Attached to the chiral carbon will be a photocleavableleaving group, such as a carboxylic acid, a carboxylic acid ester, orthe like. Other suitable leaving groups and the structural requirementsfor their photo-lability would be known in the art. In addition, thechiral carbon may also frequently include electron withdrawing groups,such as a cyano group, to increase the lability of the leaving group.Examples of such compounds include aryl cyano acids, aryloxy aceticacids, α-aryl propionic acids, benzoin esters, and the like.

As noted, the structure, synthesis and chemistry of such compounds aredetermined substantially by the specific photochemical mechanism atwork, and would be known to those of ordinary skill in the art in viewof the instant disclosure. For example, when one desires to utilize thewell known mechanism of photodecarboxylation of an acid group, arylcyano acids, aryloxy acetic acids and arylcyano acetic acids arecommonly employed. These well know photochemical techniques as well asother photochemical techniques suitable for adaptation to the presentinvention are well understood in the field of synthetic organicchemistry as discussed in, for example, Coyle, J. D., Introduction toOrganic Photochemistry, John Wiley & Sons (1986) (Chapter 4, pp107-111); Cameron et al., J. Am. Chem. Soc., Vol. 113, No. 11, pp4303-4313 (1991); Sonawabe et al., Tetrahedron Asymmetry, Vol. 3, No. 2,pp. 163-192 (1992); and Davidson et al., J. Chem. Soc. Perkin II, p 1357(1972), all of which are incorporated herein by reference. Those ofordinary skill in the art would be able to select and synthesizeappropriate photo-tunable chiral dopants within this category in view ofthe instant disclosure.

Preferred photo-tunable chiral dopants that undergo triplet sensitizedracemization and/or isomerization useful for the practice of thisinvention include, but are not limited to, those shown below in Table 2:

TABLE 2 Photo-Tunable Chiral Dopants PTCD-1

PTCD-2

PTCD-3

PTCD-4

PTCD-5

PTCD-6

PTCD-7

PTCD-8

PTCD-9

PTCD-10

PTCD-11

Since the chemistry of photodecarboxylation is one of the betterunderstood mechanisms, compounds of this category having aphotocleavable carboxylic acid group are particularly suitable foradaptation to use as photo-tunable chiral dopants. In these compounds,the chiral carbon may be either directly alpha to the ring or ringsystem, such as in the arylcyano acetic acid, or may be beta to the ringor ring system through a bond or heteroatom. In these photochemicalreactions, the leaving group is a photocleavable carboxylic acid or acidester, the lability of which may be enhanced by the inclusion on thechiral center of an electron withdrawing group such as a cyano group, orby forming the chiral carbon beta to the ring system through aheteroatom such as oxygen or sulfur, as in the formulas above. Upontriplet-sensitized irradiation, the acid moiety is eliminated as CO₂thereby destroying the chirality of the compound.

Aryl ketones are known to undergo triplet sensitized bond cleavagereactions and are useful as triplet sensitized photo-tunable chiraldopants. These molecules undergo triplet sensitized bond cleavage of thechiral center to destroy the chirality, wherein the chiral carbon isalpha to the keto group. The keto group is bound to an aromatic ring orring system to enhance the lability of the alpha leaving group. Thesecompounds employ the well know mechanism of alpha cleavage of the carbonalpha to the aryl ketone and are especially preferred. Thephotochemistry of alpha-cleavage has been known for decades and isreadily adapted to the claimed utility by those of ordinary skill in theart in view of the instant disclosure. For example, the photochemistryof the alpha-cleavage is discussed at length in Chapter 13 of Turro,Modern Molecular Photochemistry, The Benjamin/Cummings Publishing Co.,Inc., (1978), and the footnotes therein, and by Lewis et al., J. Am.Chem. Soc. 95:18, pp 5973-76 (1973), all of which are incorporatedherein by reference. The essence of this chemistry is in thephoto-cleavability of a leaving group in the alpha position to acarbonyl, in particular, alpha to a ketone group. The susceptibility ofalpha leaving group to photo-cleavage is most pronounced when the ketogroup is directly attached to an aromatic ring, which provides anexcellent electron withdrawing sink rendering the alpha-leaving group(beta to the ring) particularly labile. Hence, especially preferredcompounds according to this embodiment are aryl ketones.

When the alpha carbon is a chiral center, such compounds will havephoto-tunable chirality. Accordingly, these compounds are excellentadditives to adjust the chirality of a liquid crystal composition. Upontriplet-sensitized irradiation, the bond between the keto carbon andalpha carbon is easily cleaved, eliminating the chiral center andthereby destroying the chirality of the molecule and the leaving group.Therefore, the preferred alpha-cleavable photo-tunable chiral dopantsare aromatic ketones generally characterized by a keto group directlyattached to an aromatic ring or ring system, and a chiral center alphato the keto group. The aromatic ring or ring system may be mesogenic,although this is not a requirement. Suitable aromatic groups includesubstituted or unsubstituted phenyls, biphenyls, aryls, heteroaryls andthe like. The synthesis of compounds having these minimum requirementswould be well known to those of ordinary skill in the art. It iscontemplated that virtually any such compounds fulfilling these minimumrequirements will have triplet sensitized photo-tunable chirality and besuitable for use in the instant invention.

In addition to photo-tunable chiral dopants, the cholesteric liquidcrystal material may further comprises one or more additional chiraldopants that do not show a substantial change of chirality, but insteadretain their chirality, under the same conditions where thephoto-tunable chiral dopant loses its chirality. Thus, the additionalchiral dopants should retain their chirality when irradiation of thetriplet sensitizer results in complete loss of chirality inphoto-tunable chiral dopant. These additional chiral dopants arehereinafter also referred to as ‘non-photo-tunable’ chiral dopants.

The non-photo-tunable chiral dopant added to the nematic mixture toinduce the helical twisting of the mesophase, thereby allowingreflection of visible light, can be of any useful structural class. Thechoice of dopant depends upon several characteristics including amongothers its chemical compatibility with the nematic host, helicaltwisting power, temperature sensitivity, and light fastness. Many chiraldopant classes are known in the art: e.g., G. Gottarelli and G. Spada,Mol. Cryst. Liq. Crys., 123, 377 (1985); G. Spada and G. Proni,Enantiomer, 3, 301 (1998) and references therein, incorporated herein byreference. Typical well known dopant classes include 1,l-binaphtholderivatives; isosorbide and similar isomannide esters as disclosed inU.S. Pat. No. 6,217,792; TADDOL derivatives as disclosed in U.S. Pat.No. 6,099,751; and the pending spiroindanes esters as disclosed in U.S.patent application Ser. No. 10/651,692 by T. Welter et al., filed Aug.29, 2003, now U.S. Pat. No. 7,052,743, titled “Chiral Compounds AndCompositions Containing The Same,” hereby incorporated by reference.

The triplet sensitizer (S) used in the invention initiates theracemization, either by bond cleavage or isomerization, of the reactantphoto-tunable chiral dopant following absorption of actinic radiation.The sensitized racemization process is illustrated in Scheme 1, below.The process that produces the racemization of the photo-tunable chiraldopant takes place in the lowest excited triplet state of thephoto-tunable chiral dopant (³PTCD). It involves a racemization of ³PTCDto produce an achiral product P that may or may not be in its lowesttriplet excited state (³P). If the product is in the triplet excitedstate, ³P can subsequently transfer its energy to another molecule ofphoto-tunable chiral dopant producing more ³P, which can in turnisomerize to additional ³P to result in a chain process. This results ina very highly efficient process, characterized by high quantumefficiency. Energy loss from ³PTCD to ground state photo-tunable chiraldopant or so-called intersystem crossing of ³PTCD to ground statephoto-tunable chiral dopant may also occur, which in effect terminatesthe process.

The sensitizer must be capable of producing ³PTCD by transferring energyfrom its own lowest triplet excited state (³S) after the sensitizer hasabsorbed light. To be effective in producing ³PTCD, the lowest tripletenergy of said photo-reactive chiral compound is lower than the lowesttriplet energy of said triplet sensitizer. If the lowest triplet energyof the sensitizer is lower than the lowest triplet energy of thephoto-reactive chiral compound, the lowest triplet energy of thesensitizer may be no more than about 4-6 kcal/mole below that of thereactant (photo-reactive chiral compound). More preferably, the tripletenergy of the sensitizer is at least as high as that of the reactant.

Furthermore, it is important that, upon absorption of light, thesensitizer yields ³S efficiently. Since the absorption of light by thesensitizer generally produces an excited singlet state of the sensitizer(¹S), the ¹S state must first undergo so-called intersystem crossing toproduce ³S, which initiates the isomerization through triplet energytransfer to the photo-tunable chiral dopant.

The amount of sensitizer used in the optical recording material of thisinvention depends largely on its optical density at the wavelength(s) oflight used to sensitize the isomerization. Solubility of the sensitizermay also be a factor. In a nematic liquid crystal material, thesensitizer will generally comprise from 0.002 to 20% by weight of thephoto-tunable chiral dopant of this invention. The sensitizer may alsobe covalently attached to the photo-tunable chiral dopant of thisinvention. Either a polymer bound sensitizer or a monomeric sensitizerwith a low extinction coefficient may be utilized at relatively highlevels. Such high levels may help facilitate the transfer of tripletenergy.

Since this invention relies on a triplet sensitization process, thesensitizer must have a reasonably high intersystem crossing quantumyield for the formation of ³S on absorption of light. Preferably, theintersystem crossing quantum yield of a sensitizer of this invention isat least 0.2. Most preferably, the intersystem crossing quantum yield ofa sensitizer of this invention is at least 0.9.

The triplet energies of the sensitizers of this invention may beobtained in a variety of ways. Energies for some sensitizers or closelyrelated analogs are disclosed in the prior art. For most sensitizers,the lowest triplet state energies may be obtained from low-temperature(e.g., 77° K.) phosphorescence spectra. The sensitizer is typicallydissolved in a solvent (such as ethyl acetate) or a mixture of solventsand the solution is placed in an optical cell and immersed in liquidnitrogen. The sensitizer is then excited with light at a wavelengthwhere it absorbs, and its phosphorescence spectrum is measured. Thehighest energy absorption band (the so-called 0-0 band) in thephosphorescence spectrum can usually be taken as the energy of thelowest triplet state of sensitizer. For sensitizers with weak orobscured emission or in which the ground state and lowest triplet statehave substantial differences in geometry, triplet energies can beobtained either from rates of energy transfer from a series of moleculeswith known triplet energies or from measured equilibria with triplets ofknown energies. The former procedure is described in J. Amer. Chem. Soc.102, 2152 (1980) and the latter procedure is described in J. Phys. Chem.78, 196 (1974).

In nematic liquid crystal materials, sensitizers and photo-tunablechiral dopants can occupy sites of different polarity, such that exacttriplet energies are site dependent. To the extent that this is true forthe sensitizers, cosensitizers (see below for identification) andphoto-tunable chiral dopants in this invention, the reported tripletenergies represent approximate or average values.

The ketocoumarins disclosed in Tetrahedron 38, 1203 (1982) represent oneclass of sensitizers useful for the practice of this invention. Otherclasses of sensitizers useful for the practice of this invention includexanthones, thioxanthones, arylketones and polycyclic aromatichydrocarbons. Especially preferred are sensitizers that absorbultraviolet light above 400 nm or visible light.

Specific sensitizers useful for the practice of this invention include,but are not limited to, those shown below:

TABLE 1 Triplet Sensitizers S-1

S-2

S-3

S-4

S-5

S-6

S-7

S-8

S-9

S-10

S-11

S-12

S-13

S-14

S-15

S-16

S-17

S-18

S-19

S-20

S-21

S-22

Entry S-22 in Table 1 is an example of a sensitizer bearing apolymerizable vinyl group, and are therefore a suitable sensitizer forcovalent attachment to the polymeric matrix.

Triplet energies of sensitizers S-1, S-2, S-3, S-4, S-5, S-6, S-7, S-8and S-10 have been measured (via phosphorescence) in ethyl acetate as62, 64, 56, 63, 57.5, 50.5, 57, 60.5 56.5 and 56 kcal/mole,respectively. The triplet energies of the reactants noted above weremeasured by the procedure described in J. Amer. Chem. Soc. 102, 2152(1980).

The nematic liquid crystal material of this invention may furthercomprise a so-called cosensitizer (CS) to assist in the transfer oftriplet energy to photo-tunable chiral dopant. The transfer of tripletenergy occurs efficiently only over a short distance (about 1.0-1.5 nm).If a cosensitizer with a triplet energy not too far above that of thesensitizer is added to the optical recording material, the cosensitizermay serve as a bridge in the transfer of triplet energy from ³S to PTCD,Scheme 2, even after a shell of product has formed around thesensitizer. Like the sensitizer, the cosensitizer should not have atriplet energy more than about 6 kcal/mole below the triplet energy ofthe reactant, and preferably not more than 4 kcal/mole below the tripletenergy of the reactant.

The cosensitizer (CS) can also assist in the transfer of triplet energyfrom ³PTCD to PTCD, Scheme 2, as long as the cosensitizer has tripletenergy not significantly greater than the triplet energy of P. Whiledependent on structure, the ³P energies are in the range of about 50-76kcal/mole for the P of this invention. Thus, a cosensitizer of thisinvention must generally have a triplet energy in the range of about45-72 kcal/mole. To be effective in producing ³PTCD, the lowest tripletenergy of said photo-reactive chiral compound is lower than the lowesttriplet energy of said triplet cosensitizer. If the lowest tripletenergy of the cosensitizer is lower than the lowest triplet energy ofthe photo-reactive chiral compound, the lowest triplet energy of thecosensitizer may be no more than about 4-6 kcal/mole below that of thereactant (photo-reactive chiral compound). The cosensitizer should notabsorb more than about 10 percent of the actinic radiation absorbed bythe sensitizer, if it is to function as described. Otherwise, it is moreproperly considered as a sensitizer itself. If either a sensitizer or acosensitizer produces excessive light absorption at the sensitizationwavelength, it can restrict reaction of photo-tunable chiral dopant to arelatively thin layer near the exposed surface and reduce overallquantum yields due to inefficient light penetration through the rest ofthe sample.

The cosensitizer may be monomeric or it may be covalently attached to apolymeric matrix. Incorporating the cosensitizer as part of the polymercan allow high concentrations of cosensitizer and increase isomerizationquantum efficiencies. A monomeric cosensitizer may comprise from about 2to 20 percent by weight of the optical recording material. If covalentlyattached to the polymeric matrix, the cosensitizer may comprise fromabout 2 to 90 percent of the optical recording material.

Modern chiral nematic liquid crystal materials usually include at leastone nematic host combined with a chiral dopant. In general, the nematicliquid crystal phase is composed of one or more mesogenic componentscombined to provide useful composite properties. Many such materials areavailable commercially. The nematic component of the chiral nematicliquid crystal mixture may be comprised of any suitable nematic liquidcrystal mixture or composition having appropriate liquid crystalcharacteristics. Nematic liquid crystals suitable for use in the presentinvention are preferably composed of compounds of low molecular weightselected from nematic or nematogenic substances, for example from theknown classes of the azoxybenzenes, benzylideneanilines, biphenyls,terphenyls, phenyl or cyclohexyl benzoates, phenyl or cyclohexyl estersof cyclohexanecarboxylic acid; phenyl or cyclohexyl esters ofcyclohexylbenzoic acid; phenyl or cyclohexyl esters ofcyclohexylcyclohexanecarboxylic acid; cyclohexylphenyl esters of benzoicacid, of cyclohexanecarboxylic acid and ofcyclohexylcyclohexanecarboxylic acid; phenyl cyclohexanes;cyclohexylbiphenyls; phenyl cyclohexylcyclohexanes;cyclohexylcyclohexanes; cyclohexylcyclohexenes;cyclohexylcyclohexylcyclohexenes; 1,4-bis-cyclohexylbenzenes;4,4-bis-cyclohexylbiphenyls; phenyl- or cyclohexylpyrimidines; phenyl-or cyclohexylpyridines; phenyl- or cyclohexylpyridazines; phenyl- orcyclohexyldioxanes; phenyl- or cyclohexyl-1,3-dithianes;1,2-diphenylethanes; 1,2-dicyclohexylethanes;1-phenyl-2-cyclohexylethanes;1-cyclohexyl-2-(4-phenylcyclohexyl)ethanes;1-cyclohexyl-2′,2-biphenylethanes; 1-phenyl-2-cyclohexylphenylethanes;optionally halogenated stilbenes; benzyl phenyl ethers; tolanes;substituted cinnamic acids and esters; and further classes of nematic ornematogenic substances. The 1,4-phenylene groups in these compounds mayalso be laterally mono- or difluorinated. The liquid crystallinematerial of this preferred embodiment is based on the achiral compoundsof this type. The most important compounds, that are possible ascomponents of these liquid crystalline materials, can be characterizedby the following formula R′—X—Y-Z-R″ wherein X and Z, which may beidentical or different, are in each case, independently from oneanother, a bivalent radical from the group formed by -Phe-, -Cyc-,-Phe-Phe-, -Phe-Cyc-, -Cyc-Cyc-, -Pyr-, -Dio-, -B-Phe- and -B-Cyc-;wherein Phe is unsubstituted or fluorine substituted 1,4-phenylene, Cycis trans-1,4-cyclohexylene or 1,4-cyclohexenylene, Pyr ispyrimidine-2,5-diyl or pyridine-2,5-diyl, Dio is 1,3-dioxane-2,5-diyl,and B is 2-(trans-1,4-cyclohexyl)ethyl, pyrimidine-2,5-diyl,pyridine-2,5-diyl or 1,3-dioxane-2,5-diyl. Y in these compounds isselected from the following bivalent groups —CH═CH—, —C≡C—, —N═N(O)—,—CH═CY′—, —CH═N(O)—, —CH2-CH2-, —CO—O—, —CH2-O—, —CO—S—, —CH2-S—,—COO-Phe-COO— or a single bond, with Y′ being halogen, preferablychlorine, or CN; R′ and R″ are, in each case, independently of oneanother, alkyl, alkenyl, alkoxy, alkenyloxy, alkanoyloxy, alkoxycarbonylor alkoxycarbonyloxy with 1 to 18, preferably 1 to 12 C atoms, oralternatively one of R′ and R″ is —F, —CF3, —OCF3, —Cl, —NCS or CN. Inmost of these compounds R′ and R′ are, in each case, independently ofeach another, alkyl, alkenyl or alkoxy with different chain length,wherein the sum of C atoms in nematic media generally is between 2 and9, preferably between 2 and 7. The nematic liquid crystal phasestypically consist of 2 to 20, preferably 2 to 15 components. The abovelist of materials is not intended to be exhaustive or limiting. Thelists disclose a variety of representative materials suitable for use ormixtures, which comprise the active element in electro-optic liquidcrystal compositions.

Suitable chiral nematic liquid crystal compositions preferably have apositive dielectric anisotropy and include chiral material in an amounteffective to form focal conic and twisted planar textures. Chiralnematic liquid crystal materials are preferred because of theirexcellent reflective characteristics, bistability and gray scale memory.The chiral nematic liquid crystal is typically a mixture of nematicliquid crystal and chiral material in an amount sufficient to producethe desired pitch length. Suitable commercial nematic liquid crystalsinclude, for example, E7, E44, E48, E31, E80, BL087, BL101, ZLI-3308,ZLI-3273, ZLI-5048-000, ZLI-5049-100, ZLI-5100-100, ZLI-5800-000,MLC-6041-100.TL202, TL203, TL204 and TL205 manufactured by E. Merck(Darmstadt, Germany). Although nematic liquid crystals having positivedielectric anisotropy, and especially cyanobiphenyls, are preferred,virtually any nematic liquid crystal known in the art, including thosehaving negative dielectric anisotropy should be suitable for use in theinvention. Other nematic materials may also be suitable for use in thepresent invention as would be appreciated by those skilled in the art.

As used herein, a “liquid crystal display” (LCD) is a type of flat paneldisplay used in various electronic devices. At a minimum, an LCDcomprises a substrate, at least one conductive layer and a liquidcrystal layer. The liquid crystal (LC) is used as an optical switch. Thesubstrates are usually manufactured with transparent, conductiveelectrodes, in which electrical “driving” signals are coupled. Thedriving signals induce an electric field which can cause a phase changeor state change in the liquid crystal material, the liquid crystalexhibiting different light reflecting characteristics according to itsphase and/or state.

In one embodiment, at least one imageable layer is applied to thesupport. The imageable layer can contain an electrically imageablematerial. The electrically imageable material can be light emitting orlight modulating. Especially preferred are chiral nematic liquidcrystals. The chiral nematic liquid crystals can be polymer dispersedliquid crystals (PDLC). Structures having stacked imaging layers ormultiple support layers, however, are optional for providing additionaladvantages in some case.

The liquid crystalline material preferred are chiral nematic liquidcrystals. The chiral nematic liquid crystals can be polymer dispersedliquid crystals (PDLC). Structures having stacked imaging layers ormultiple support layers, however, are optional for providing additionaladvantages in some case

In a preferred embodiment, the electrically imageable material can beaddressed with an electric field and then retain its image after theelectric field is removed, a property typically referred to as“bistable”. Especially preferred are chiral nematic liquid crystals. Thechiral nematic liquid crystals can be polymer dispersed liquid crystals(PDLC).

Chiral nematic liquid crystal refers to the type of liquid crystalhaving finer pitch than that of twisted nematic and super-twistednematic used in commonly encountered liquid crystal devices. Chiralnematic liquid crystals are so named because such liquid crystalformulations are commonly obtained by adding chiral agents to hostnematic liquid crystals. Chiral nematic liquid crystals may be used toproduce bistable or multi-stable displays. These devices havesignificantly reduced power consumption due to their nonvolatile“memory” characteristic. Since such displays do not require a continuousdriving circuit to maintain an image, they consume significantly reducedpower. Chiral nematic displays are bistable in the absence of a field;the two stable textures are the reflective planar texture and the weaklyscattering focal conic texture. In the planar texture, the helical axesof the chiral nematic liquid crystal molecules are substantiallyperpendicular to the substrate upon which the liquid crystal isdisposed. In the focal conic state the helical axes of the liquidcrystal molecules are generally randomly oriented. Adjusting theconcentration of chiral dopants in the chiral nematic material modulatesthe pitch length of the mesophase and, thus, the wavelength of radiationreflected. Chiral nematic materials that reflect infrared radiation andultraviolet have been used for purposes of scientific study. Commercialdisplays are most often fabricated from chiral nematic materials thatreflect visible light. Some known LCD devices include chemically etched,transparent, conductive layers overlying a glass substrate as describedin U.S. Pat. No. 5,667,853, incorporated herein by reference.

The preferred liquid crystal material is a chiral liquid crystalmaterials having positive dielectric anisotropy and including chiraldopant in an amount effective to form focal conic and twisted planartextures. Chiral nematic liquid crystal materials are preferred becauseof their excellent reflective characteristics, bistability and grayscale memory. The chiral nematic liquid crystal is typically a mixtureof nematic liquid crystal and chiral material in an amount sufficient toproduce the desired pitch length, which can thereafter be modified bythe photo-tunable chiral dopant additive. Suitable nematic liquidcrystals include, for example, E7, E48, E31, E80, TL202, TL203, TL204and TL205 manufactured by E. Merck. Although nematic liquid crystalshaving positive dielectric anisotropy, and especially cyanobiphenyls,are preferred, virtually any nematic liquid crystal known in the art,including those having negative dielectric anisotropy, should besuitable for use in the invention. Suitable chiral dopants include, forexample, CB15, CE2, CE1, R1101 and TM74A, also manufactured by E. Merck.Other chiral nematic or cholesteric liquid crystals and liquid crystalmixtures suitable for use in the invention would be known to those ofordinary skill in the art in view of the instant disclosure.

In one embodiment, a chiral nematic liquid crystal composition may bedispersed in a continuous matrix. Such materials are referred to as“polymer dispersed liquid crystal” materials or “PDLC” materials. Suchmaterials can be made by a variety of methods. For example, Doane et al.(Applied Physics Letters, 48, 269 (1986)) disclose a polymer disperseliquid crystal comprising approximately 0.4 μm droplets of nematicliquid crystal 5CB in a polymer binder. A phase separation method isused for preparing the polymer disperse liquid crystal. A solutioncontaining monomer and liquid crystal is filled in a display cell andthe material is then polymerized. Upon polymerization the liquid crystalbecomes immiscible and nucleates to form droplets. West et al. (AppliedPhysics Letters 63, 1471 (1993)) disclose a polymer disperse liquidcrystal comprising a chiral nematic mixture in a polymer binder. Onceagain a phase separation method is used for preparing the polymerdisperse liquid crystal. The liquid crystal material and polymer (ahydroxy functionalized polymethylmethacrylate) along with a crosslinkerfor the polymer are dissolved in a common organic solvent toluene andcoated on an indium tin oxide (ITO) substrate. A dispersion of theliquid crystal material in the polymer binder is formed upon evaporationof toluene at high temperature. The phase separation methods of Doane etal. and West et al. require the use of organic solvents that may beobjectionable in certain manufacturing environments.

The contrast of the display is degraded if there is more than asubstantial monolayer of N*LC domains. The term “substantial monolayer”is defined by the Applicants to mean that, in a direction perpendicularto the plane of the display, there is no more than a single layer ofdomains sandwiched between the electrodes at most points of the display(or the imaging layer), preferably at 75 percent or more of the points(or area) of the display, most preferably at 90 percent or more of thepoints (or area) of the display. In other words, at most, only a minorportion (preferably less than 10 percent) of the points (or area) of thedisplay has more than a single domain (two or more domains) between theelectrodes in a direction perpendicular to the plane of the display,compared to the amount of points (or area) of the display at which thereis only a single domain between the electrodes.

One preferred embodiment, described in U.S. Patent Publication No.2006/0134565, incorporated herein by reference utilizes a high contrastreflective display comprising at least one substrate, at least oneelectrically conductive layer and at least one close-packed, orderedmonolayer of domains of electrically modulated material in a fixed,preferably crosslinked, polymer matrix and a method of making the same.The electrically modulated material is preferred to be a chiral nematicliquid crystal incorporated in a polymer matrix. Chiral nematic liquidcrystalline materials may be used to create electronic displays that areboth bistable and viewable under ambient lighting. Furthermore, theliquid crystalline materials may be dispersed as micron sized dropletsin an aqueous medium, mixed with a suitable binder material and coatedon a flexible conductive support to create potentially low costdisplays. The operation of these displays is dependent on the contrastbetween the planar reflecting state and the weakly scattering focalconic state. In order to derive the maximum contrast from thesedisplays, it is desired that the chiral nematic liquid crystal domainsor droplets are spread on a conductive support as a close-packed orderedmonolayer. It is possible to prepare such an ordered monolayer by firstapplying an aqueous dispersion of chiral nematic liquid crystal domainsto the substrate in the presence of a suitable binder, allowing thedomains or droplets to self-assemble into a close-packed orderedmonolayer, preferably a hexagonal close-packed (HCP) monolayer and thenallowing the binder material to set, become fixed or crosslink topreserve the close-packed ordered monolayer structure so that otheraqueous layers can be spread above the imaging layer without affectingthe close-packed structure.

The amount of material needed for a monolayer can be accuratelydetermined by calculation based on individual domain size, assuming afully closed packed arrangement of domains. (In practice, there may beimperfections in which gaps occur and some unevenness due to overlappingdroplets or domains.) On this basis, the calculated amount is preferablyless than about 150 percent of the amount needed for monolayer domaincoverage, preferably not more than about 125 percent of the amountneeded for a monolayer domain coverage, more preferably not more than110 percent of the amount needed for a monolayer of domains.Furthermore, improved viewing angle and broadband features may beobtained by appropriate choice of differently doped domains based on thegeometry of the coated droplet and the Bragg reflection condition.

In one preferred embodiment of the invention, the display device ordisplay sheet has simply a single imaging layer of liquid crystalmaterial along a line perpendicular to the face of the display,preferably a single layer coated on a flexible substrate. Such asstructure, as compared to vertically stacked imaging layers each betweenopposing substrates, is especially advantageous for monochrome shelflabels and the like. Structures having stacked imaging layers, however,are optional for providing additional advantages in some case.

Preferably, the domains are flattened spheres and have on average athickness substantially less than their length, preferably at least 50%less. More preferably, the domains on average have a thickness (depth)to length ratio of 1:2 to 1:6. The flattening of the domains can beachieved by proper formulation and sufficiently rapid drying of thecoating. The domains preferably have an average diameter of 2 to 30microns. The imaging layer preferably has a thickness of 10 to 150microns when first coated and 2 to 20 microns when dried.

The flattened domains of liquid crystal material can be defined ashaving a major axis and a minor axis. In a preferred embodiment of adisplay or display sheet, the major axis is larger in size than the cell(or imaging layer) thickness for a majority of the domains. Such adimensional relationship is shown in U.S. Pat. No. 6,061,107, herebyincorporated by reference in its entirety.

Liquid crystal domains may be preferably made using a limitedcoalescence methodology, as disclosed in U.S. Pat. Nos. 6,556,262 and6,423,368, incorporated herein by reference. Limited coalescence isdefined as dispersing a light modulating material below a given size,and using coalescent limiting material to limit the size of theresulting domains. Such materials are characterized as having a ratio ofmaximum to minimum domain size of less than 2:1. By use of the term“uniform domains”, it is meant that domains are formed having a domainsize variation of less than 2:1. Limited domain materials have improvedoptical properties.

An immiscible, field responsive light modulating material along with aquantity of colloidal particles is dispersed in an aqueous system andblended to form a dispersion of field responsive, light modulatingmaterial below a coalescence size. When the dispersion, also referred toherein as an emulsion, is dried, a coated material is produced which hasa set of uniform domains having a plurality of electrically responsiveoptical states. The colloidal solid particle, functioning as anemulsifier, limits domain growth from a highly dispersed state.Uniformly sized liquid crystal domains are created and machine coated tomanufacture light modulating, electrically responsive sheets withimproved optical efficiency.

Specifically, a liquid crystal material comprising a triplet sensitizerand at least one photo-tunable chiral dopant may be dispersed an aqueousbath containing a water soluble binder material such as deionizedgelatin, polyvinyl alcohol (PVA) or polyethylene oxide (PEO). Suchcompounds are machine coatable on equipment associated with photographicfilms. Preferably, the binder has a low ionic content, as the presenceof ions in such a binder hinders the development of an electrical fieldacross the dispersed liquid crystal material. Additionally, ions in thebinder can migrate in the presence of an electrical field, chemicallydamaging the light modulating layer. The liquid crystal/gelatin emulsionis coated to a thickness of between 5 and 30 microns to optimize opticalproperties of light modulating layer. The coating thickness, size of theliquid crystal domains, and concentration of the domains of liquidcrystal materials are designed for optimum optical properties.

In an exemplary embodiment, a liquid crystalline material comprising atriplet sensitizer and one or more photo-tunable chiral dopant andadditional chiral dopants is homogenized in the presence of finelydivided silica, a coalescence limiting material, (LUDOX® from duPontCorporation). A promoter material, such as a copolymer of adipic acidand 2-(methylamino) ethanol, is added to the aqueous bath to drive thecolloidal particles to the liquid-liquid interface. The liquid crystalmaterial is dispersed using ultrasound to create liquid crystal domainsbelow 1 micron in size. When the ultrasound energy was removed, theliquid crystal material coalesced into domains of uniform size. Theratio of smallest to largest domain size varied by approximately 1:2. Byvarying the amount of silica and copolymer relative to the liquidcrystalline material, uniform domain size emulsions of average diameter(by microscopy) approximately 1, 3, and, 8 micron were produced. Theseemulsions were diluted into gelatin solution for subsequent coating.

Domains of a limited coalescent material maintain their uniform sizeafter the addition of the surfactant and after being machine coated.There were few, if any, parasitic domains having undesirableelectro-optical properties within the dried coatings produced by thelimited coalescence method. Coatings made using limited coalescencehaving a domain size of about 2 microns may have the greatesttranslucence. For constant material concentrations and coatingthickness, limited coalescent materials having a domain size of about 2microns in size are significantly more translucent than any sizeddomains formed using conventional dispersion.

Sheets made by the limited coalescence process have curves similar tothose of conventionally dispersed materials. However, with 8 to 10micron domains, the material may demonstrate reduced scattering due tothe elimination of parasitic domains. Conventionally dispersedcholesteric materials contain parasitic domains, which reflect light inwavelengths outside the wavelengths reflected by the cholestericmaterial. Limited coalescent dispersions have reduced reflection inother wavelengths due to the elimination of parasitic domains. Theincreased purity of color is important in the development of full colordisplays requiring well separated color channels to create a full colorimage. Limited coalescent cholesteric materials provide purer lightreflectance than cholesteric liquid crystal material dispersed byconventional methods. Such materials may be produced using conventionalphotographic coating machinery.

In order to provide suitable formulations for applying a layercontaining the liquid crystal domains, the dispersions are combined witha hydrophilic colloid, gelatin being the preferred material. Surfactantsmay be included with the liquid crystal dispersion prior to the additionof gelatin in order to prevent the removal of the particulate suspensionstabilizing agent from the droplets. This aids in preventing furthercoalescence of the droplets.

As for the suspension stabilizing agents that surround and serve toprevent the coalescence of the droplets, any suitable colloidalstabilizing agent known in the art of forming polymeric particles by theaddition reaction of ethylenically unsaturated monomers by the limitedcoalescence technique can be employed, such as, for example, inorganicmaterials such as, metal salt or hydroxides or oxides or clays, organicmaterials such as starches, sulfonated crosslinked organic homopolymersand resinous polymers as described, for example, in U.S. Pat. No.2,932,629; silica as described in U.S. Pat. No. 4,833,060; copolymerssuch as copoly(styrene-2-hydroxyethyl methacrylate-methacrylicacid-ethylene glycol dimethacrylate) as described in U.S. Pat. No.4,965,131, all of which are incorporated herein by reference. Silica isthe preferred suspension stabilizing agent.

Suitable promoters to drive the suspension stabilizing agent to theinterface of the droplets and the aqueous phase include sulfonatedpolystyrenes, alginates, carboxymethyl cellulose, tetramethyl ammoniumhydroxide or chloride, triethylphenyl ammonium hydroxide, triethylphenylammonium hydroxide, triethylphenyl ammonium chloride,diethylaminoethylmethacrylate, water soluble complex resinous aminecondensation products, such as the water soluble condensation product ofdiethanol amine and adipic acid, such as poly(adipicacid-co-methylaminoethanol), water soluble condensation products ofethylene oxide, urea, and formaldehyde and polyethyleneimine; gelatin,glue, casein, albumin, gluten, and methoxycellulose. The preferredpromoter is triethylphenyl ammonium chloride.

In order to prevent the hydrophilic colloid from removing the suspensionstabilizing agent from the surface of the droplets, suitable anionicsurfactants may be included in the mixing step to prepare the coatingcomposition such as polyisopropyl naphthalene-sodium sulfonate, sodiumdodecyl sulfate, sodium dodecyl benzene sulfonate, as well as thoseanionic surfactants set forth in U.S. Pat. No. 5,326,687 and in SectionXI of Research Disclosure 308119, December 1989, entitled “PhotographicSilver Halide Emulsions, Preparations, Addenda, Processing, andSystems”, both of which are incorporated herein by reference. Aromaticsulfonates are more preferred and polyisopropyl naphthalene sulfonate ismost preferred.

Suitable hydrophilic binders include both naturally occurring substancessuch as proteins, protein derivatives, cellulose derivatives (e.g.cellulose esters), gelatins and gelatin derivatives, polysaccaharides,casein, and the like, and synthetic water permeable colloids such aspoly(vinyl lactams), acrylamide polymers, poly(vinyl alcohol) and itsderivatives, hydrolyzed polyvinyl acetates, polymers of alkyl andsulfoalkyl acrylates and methacrylates, polyamides, polyvinyl pyridine,acrylic acid polymers, maleic anhydride copolymers, polyalkylene oxide,methacrylamide copolymers, polyvinyl oxazolidinones, maleic acidcopolymers, vinyl amine copolymers, methacrylic acid copolymers,acryloyloxyalkyl acrylate and methacrylates, vinyl imidazole copolymers,vinyl sulfide copolymers, and homopolymer or copolymers containingstyrene sulfonic acid. Gelatin is preferred.

An especially preferred class of liquid crystal materials with which theinventive combination of a triplet sensitizer and a photo-tunable chiraldopant additive may be used include polymer, the polymer beingdistributed in the finished cell in a polymer network in an amount thatprovides a stabilizing or constraining effect on the pixels. In somecells, the polymer also serves to stabilize the focal conic and twistedplanar textures in the absence of a field. The material used to form thepolymer network is preferably soluble with the chiral nematic liquidcrystal and phase separates upon solidification to form phase separatedpolymer domains. Suitable polymer materials include UV curable,thermoplastic and thermosetting polymers. Examples of suitable materialsinclude those formed from monomers having at least two polymerizabledouble bonds, polymethylmethacrylates, bisacrylates, vinyl ethers,hydroxyfunctionalized polymethacrylates, urethanes, and epoxy systems toname a few. Other suitable materials would be known to those of ordinaryskill in the art in view of the present disclosure. The amount ofpolymer to be used depends upon the polymer, liquid crystal, tripletsensitizer and photo-tunable chiral dopant. Useful results may beobtained with polymer contents ranging from about 0.1 to about 50% byweight based on the combined weight of polymer, chiral nematic liquidcrystal and photo-tunable chiral dopant. For example, cells may beprepared with a polymer content ranging from about 0.1% to 50% usingcertain bisacrylates, from about 2 to 40% using certain hydroxyfunctionalized polymethacrylates, and about 40% when certain epoxies,thermoplastics and U.V. cured polymers are used. Preferably, the polymercontent is kept low, below about 20% and more preferably below about10%. This reduces the effect of any difference between the index ofrefraction of the polymer and the indices of refraction of the liquidcrystal, which gives rise to “haze”. Accordingly, when the polymercontent is kept low the effect of any mismatch between the indices ofrefraction of the liquid crystal and polymer is minimized. It is to beunderstood, therefore, that the polymer content is subject to somevariation, in as much as what constitutes a desirable or undesirableappearance of the cell in its various optical states is a matter ofsubjective judgment, and the need to prevent or limit diffusion frompixel to pixel may vary.

Chiral nematic liquid crystal materials and cells, as well as polymerstabilized chiral nematic liquid crystals and cells, are well known inthe art and described in, for example, Yang et al., Appl. Phys. Lett.60(25) pp 3102-04 (1992); Yang et al., J. Appl. Phys. 76(2) pp 1331(1994); published International Patent Application No. PCT/US92/09367;and published International Patent Application No. PCT/US92/03504, theseare incorporated herein by reference. Although chiral nematic liquidcrystal mixtures are preferred for use in combination with photo-tunablechiral dopants and the inventive triplet sensitizers additives, it iscontemplated within the scope of the invention that the inventivephoto-tunable chiral dopant additives may be used in combination withother chiral dopants and liquid crystal materials. For example, thetriplet sensitizer additives of the invention may be used with acombination of a chiral dopant and a photo-tunable chiral dopant andnematic liquid crystal, without the need for a separate chiral additive.In such instances the chiral dopant and photo-tunable chiral dopants incombination are determinant of the chirality and pitch of the mixture.Moreover, other optional components that may be added to the liquidcrystal mixture include dyes, chiral dyes and, for example, fumed silicato adjust the stability and appearance of the various textures.

The liquid crystal composition necessary to obtain a desired startingpitch length will vary depending upon the particular liquid crystal,chiral dopant and photo-tunable chiral dopants used, as well as thedesired mode of operation. The wavelength of the light that is reflectedby the material is given by equation 1, where n is the averagerefractive index and p is the pitch length. The band width of thereflected light Δλ is given by the following equation (3):Δλ.=λΔn/n  (3)where, Δn is the birefringence of the liquid crystal. Wavelengthsbetween about 350 nm and 850 nm are in the visible spectrum. Blue lightis typically considered to have a wavelength of between about 460 and480 nm, green light between about 500 and 520 nm, yellow light betweenabout 570 and 585 nm and red light between about 630 and 700 nm.Accordingly, one of ordinary skill in the art will be able to selectappropriate materials for the invention and their relativeconcentrations based upon the refractive indices of the materialsinvolved, their twisting power and on general principles of chiraldoping of liquid crystals to obtain optimum pitches to provide a desiredcolor. Such techniques are well known in the art as taught, for example,in the manual distributed by Hoffmann-La Roche, ltd., entitled How toDope Liquid Crystal Mixtures in Order to Ensure Optimum Pitch and toCompensate the Temperature Dependence, Schadt et al., (1990), and themanual distributed by E. Merck entitled New Chiral Dopants With HighHelical Twisting Power in Nematic Liquid Crystals, Hochgesand et al.,(1989), incorporated herein by reference. Of course, what constitutes a“good” red, blue, green or yellow for a given material will also be amatter of subjective judgment and may depend upon the use to which thematerial will be put.

Accordingly, one of ordinary skill in the art will be able to selectappropriate materials for the invention and their relativeconcentrations based upon the refractive indices of the materialsinvolved, their twisting power and on general principles of chiraldoping of liquid crystals to obtain optimum pitches to provide a desiredcolor. Such techniques are well known in the art as taught, for example,in the manual distributed by Hoffmann—Using the preferred materialswherein the chiral nematic and photo-tunable chiral dopant are of thesame chirality, suitable pitch lengths for providing a good startingblue color may be obtained when the total amount of chiral material(including the photo-tunable chiral dopant) is present in an amount offrom about 20% to about 50% by weight based on the combined weight ofchiral nematic liquid crystal and photo-tunable chiral dopant.Typically, the photo-tunable chiral dopant will comprise from about 0.01to about 20% of the total chiral component of the mixture, although itcan constitute as much as 100% of the chiral component in the mixture.Preferred, is 0.1-10% total. The desired colors for the other pixels,e.g., green, yellow, red, orange etc., are then determined by theduration or amount of photoirradiation. Similarly, using the preferredmaterials wherein the chiral nematic and the photo-tunable chiral dopantare of opposite chirality, suitable pitch lengths for providing a goodstarting red color are obtained when the total amount of chiral material(including the photo-tunable chiral dopant) is present in an amount offrom about 10 to about 30% by weight based on the combined weight ofchiral nematic liquid crystal and photo-tunable chiral dopant additive.Here, the photo-tunable chiral dopant will typically comprise from about0.1 to about 50% of the total chiral component of the mixture although,as before, it can constitute the entire chiral component of the mixture.The desired colors for the other pixels are then determined by theduration of photoirradiation of the triplet sensitizer and extent ofreaction triplet sensitization induces in the photo-tunable chiraldopant.

In practical application, there may be a need to use more than oneadditive. Similarly, depending upon the application to which thematerial is to be put, it may be necessary to select differentphoto-tunable chiral dopants. For example, in order to extend the pitchchange across the entire color spectrum to produce a full color display,it is preferable to select a photo-tunable chiral dopant with a highhelical twisting power (HTP), such as a 1,1′-binaphthol. Similarly,when, for example, only blue and yellow pixels are desired one canselect a photo-tunable chiral dopant having a lower helical twistingpower. It is to be understood that the relative amounts of photo-tunablechiral dopant and chiral dopant, if any, may vary significantlydepending upon the specific materials used, their twisting power and theeffect of any polymer in the system.

The flexible plastic substrate can be any flexible self supportingplastic film that supports the thin conductive metallic film. “Plastic”means a high polymer, usually made from polymeric synthetic resins,which may be combined with other ingredients, such as curatives,fillers, reinforcing agents, colorants, and plasticizers. Plasticincludes thermoplastic materials and thermosetting materials.

The flexible plastic film must have sufficient thickness and mechanicalintegrity so as to be self supporting, yet should not be so thick as tobe rigid. Typically, the flexible plastic substrate is the thickestlayer of the composite film in thickness. Consequently, the substratedetermines to a large extent the mechanical and thermal stability of thefully structured composite film.

Another significant characteristic of the flexible plastic substratematerial is its glass transition temperature (Tg). Tg is defined as theglass transition temperature at which plastic material will change fromthe glassy state to the rubbery state. It may comprise a range beforethe material may actually flow. Suitable materials for the flexibleplastic substrate include thermoplastics of a relatively low glasstransition temperature, for example up to 150° C., as well as materialsof a higher glass transition temperature, for example, above 150° C. Thechoice of material for the flexible plastic substrate would depend onfactors such as manufacturing process conditions, such as depositiontemperature, and annealing temperature, as well as post-manufacturingconditions such as in a process line of a displays manufacturer. Certainof the plastic substrates discussed below can withstand higherprocessing temperatures of up to at least about 200° C., some up to300-350° C., without damage.

Typically, the flexible plastic substrate is polyethylene terephthalate(PET), polyethylene naphthalate (PEN), polyethersulfone (PES),polycarbonate (PC), polysulfone, a phenolic resin, an epoxy resin,polyester, polyimide, polyetherester, polyetheramide, cellulose acetate,aliphatic polyurethanes, polyacrylonitrile, polytetrafluoroethylenes,polyvinylidene fluorides, poly(methyl (x-methacrylates), an aliphatic orcyclic polyolefin, polyarylate (PAR), polyetherimide (PEI),polyethersulphone (PES), polyimide (PI), Teflon poly(perfluoro-alboxy)fluoropolymer (PFA), poly(ether ether ketone) (PEEK), poly(ether ketone)(PEK), poly(ethylene tetrafluoroethylene)fluoropolymer (PETFE), andpoly(methyl methacrylate) and various acrylate/methacrylate copolymers(PMMA). Aliphatic polyolefins may include high density polyethylene(HDPE), low density polyethylene (LDPE), and polypropylene, includingoriented polypropylene (OPP). Cyclic polyolefins may includepoly(bis(cyclopentadiene)). A preferred flexible plastic substrate is acyclic polyolefin or a polyester. Various cyclic polyolefins aresuitable for the flexible plastic substrate. Examples include Arton®made by Japan Synthetic Rubber Co., Tokyo, Japan; Zeanor T made by ZeonChemicals L.P., Tokyo Japan; and Topas® made by Celanese A. G., KronbergGermany. Arton is a poly(bis(cyclopentadiene)) condensate that is a filmof a polymer. Alternatively, the flexible plastic substrate can be apolyester. A preferred polyester is an aromatic polyester such asArylite. Although various examples of plastic substrates are set forthabove, it should be appreciated that the substrate can also be formedfrom other materials such as glass and quartz.

The flexible plastic substrate can be reinforced with a hard coating.Typically, the hard coating is an acrylic coating. Such a hard coatingtypically has a thickness of from 1 to 15 microns, preferably from 2 to4 microns and can be provided by free radical polymerization, initiatedeither thermally or by ultraviolet radiation, of an appropriatepolymerizable material. Depending on the substrate, different hardcoatings can be used. When the substrate is polyester or Arton, aparticularly preferred hard coating is the coating known as “Lintec”.Lintec contains UV cured polyester acrylate and colloidal silica. Whendeposited on Arton, it has a surface composition of 35 atom % C, 45 atom% 0, and 20 atom % Si, excluding hydrogen. Another particularlypreferred hard coating is the acrylic coating sold under the trademark“Terrapin” by Tekra Corporation, New Berlin, Wis.

In one embodiment, a sheet supports a conventional polymer dispersedlight modulating material. The sheet includes a substrate. The substratemay be made of a polymeric material, such as Kodak Estar film baseformed of polyester plastic, and have a thickness of between 20 and 200microns. For example, the substrate may be an 80 micron thick sheet oftransparent polyester. Other polymers, such as transparentpolycarbonate, can also be used. Alternatively, the substrate may bethin, transparent glass.

The LCD contains at least one conductive layer, which typically iscomprised of a primary metal oxide. This conductive layer may compriseother metal oxides such as indium oxide, titanium dioxide, cadmiumoxide, gallium indium oxide, niobium pentoxide and tin dioxide. See,Int. Publ. No. WO 99/36261 by Polaroid Corporation. In addition to theprimary oxide such as ITO, the at least one conductive layer can alsocomprise a secondary metal oxide such as an oxide of cerium, titanium,zirconium, hafnium and/or tantalum. See, U.S. Pat. No. 5,667,853 toFukuyoshi et al. (Toppan Printing Co.). Other transparent conductiveoxides include, but are not limited to ZnO₂, Zn₂SnO₄, Cd₂SnO₄, Zn₂In₂O₅,MgIn₂O₄, Ga₂O₃—In₂O₃, or TaO₃. The conductive layer may be formed, forexample, by a low temperature sputtering technique or by a directcurrent sputtering technique, such as DC-sputtering or RF-DC sputtering,depending upon the material or materials of the underlying layer. Theconductive layer may be a transparent, electrically conductive layer oftin oxide or indium-tin oxide (ITO), or polythiophene, with ITO beingthe preferred material. Typically, the conductive layer is sputteredonto the substrate to a resistance of less than 250 ohms per square.Alternatively, conductive layer may be an opaque electrical conductorformed of metal such as copper, aluminum or nickel. If the conductivelayer is an opaque metal, the metal can be a metal oxide to create alight absorbing conductive layer.

Indium tin oxide (ITO) is the preferred conductive material, as it is acost effective conductor with good environmental stability, up to 90%transmission, and down to 20 ohms per square resistivity. An exemplarypreferred ITO layer has a % T greater than or equal to 80% in thevisible region of light, that is, from greater than 400 nm to 700 nm, sothat the film will be useful for display applications. In a preferredembodiment, the conductive layer comprises a layer of low temperatureITO which is polycrystalline. The ITO layer is preferably 10-120 nm inthickness, or 50-100 nm thick to achieve a resistivity of 20-60ohms/square on plastic. An exemplary preferred ITO layer is 60-80 nmthick.

The conductive layer is preferably patterned. The conductive layer ispreferably patterned into a plurality of electrodes. The patternedelectrodes may be used to form a LCD device. In another embodiment, twoconductive substrates are positioned facing each other and cholestericliquid crystals are positioned therebetween to form a device. Thepatterned ITO conductive layer may have a variety of dimensions.Exemplary dimensions may include line widths of 10 microns, distancesbetween lines, that is, electrode widths, of 200 microns, depth of cut,that is, thickness of ITO conductor, of 100 nanometers. ITO thicknesseson the order of 60, 70, and greater than 100 nanometers are alsopossible.

The display may also contain a second conductive layer applied to thesurface of the light modulating layer. The second conductive layerdesirably has sufficient conductivity to carry a field across the lightmodulating layer. The second conductive layer may be formed in a vacuumenvironment using materials such as aluminum, tin, silver, platinum,carbon, tungsten, molybdenum, or indium. Oxides of these metals can beused to darken patternable conductive layers. The metal material can beexcited by energy from resistance heating, cathodic arc, electron beam,sputtering or magnetron excitation. The second conductive layer maycomprise coatings of tin oxide or indium-tin oxide, resulting in thelayer being transparent. Alternatively, second conductive layer may beprinted conductive ink.

For higher conductivities, the second conductive layer may comprise asilver based layer which contains silver only or silver containing adifferent element such as aluminum (Al), copper (Cu), nickel (Ni),cadmium (Cd), gold (Au), zinc (Zn), magnesium (Mg), tin (Sn), indium(In), tantalum (Ta), titanium (Ti), zirconium (Zr), cerium (Ce), silicon(Si), lead (Pb) or palladium (Pd). In a preferred embodiment, theconductive layer comprises at least one of gold, silver and agold/silver alloy, for example, a layer of silver coated on one or bothsides with a thinner layer of gold. See, Int. Publ. No. WO 99/36261 byPolaroid Corporation. In another embodiment, the conductive layer maycomprise a layer of silver alloy, for example, a layer of silver coatedon one or both sides with a layer of indium cerium oxide (InCeO). SeeU.S. Pat. No. 5,667,853, incorporated herein in by reference.

The second conductive layer may be patterned irradiating themultilayered conductor/substrate structure with ultraviolet radiation sothat portions of the conductive layer are ablated therefrom. It is alsoknown to employ an infrared (IR) fiber laser for patterning a metallicconductive layer overlying a plastic film, directly ablating theconductive layer by scanning a pattern over the conductor/filmstructure. See: Int. Publ. No. WO 99/36261 and “42.2: A New ConductorStructure for Plastic LCD Applications Utilizing ‘All Dry’ Digital LaserPatterning,” 1998 SID International Symposium Digest of TechnicalPapers, Anaheim, Calif., May 17-22, 1998, no. VOL. 29, May 17, 1998,pages 1099-1101, both incorporated herein by reference.

The LCD may also comprise at least one “functional layer” between theconductive layer and the substrate. The functional layer may comprise aprotective layer or a barrier layer. The protective layer useful in thepractice of the invention can be applied in any of a number of wellknown techniques, such as dip coating, rod coating, blade coating, airknife coating, gravure coating and reverse roll coating, extrusioncoating, slide coating, curtain coating, and the like. The liquidcrystal particles and the binder are preferably mixed together in aliquid medium to form a coating composition. The liquid medium may be amedium such as water or other aqueous solutions in which the hydrophiliccolloid are dispersed with or without the presence of surfactants. Apreferred barrier layer may acts as a gas barrier or a moisture barrierand may comprise SiOx, AlOx or ITO. The protective layer, for example,an acrylic hard coat, functions to prevent laser light from penetratingto functional layers between the protective layer and the substrate,thereby protecting both the barrier layer and the substrate. Thefunctional layer may also serve as an adhesion promoter of theconductive layer to the substrate.

In another embodiment, the polymeric support may further comprise anantistatic layer to manage unwanted charge build up on the sheet or webduring roll conveyance or sheet finishing. In another embodiment of thisinvention, the antistatic layer has a surface resistivity of from 10⁵ to10¹². Above 10¹², the antistatic layer typically does not providesufficient conduction of charge to prevent charge accumulation to thepoint of preventing fog in photographic systems or from unwanted pointswitching in liquid crystal displays. While layers greater than 10⁵ willprevent charge buildup, most antistatic materials are inherently notthat conductive and in those materials that are more conductive than10⁵, there is usually some color associated with them that will reducethe overall transmission properties of the display. The antistatic layeris separate from the highly conductive layer of ITO and provides thebest static control when it is on the opposite side of the web substratefrom that of the ITO layer. This may include the web substrate itself.

Another type of functional layer may be a color contrast layer. Colorcontrast layers may be radiation reflective layers or radiationabsorbing layers. In some cases, the rearmost substrate of each displaymay preferably be painted black. The color contrast layer may also beother colors. In another embodiment, the dark layer comprises millednonconductive pigments. The materials are milled below 1 micron to form“nano-pigments”. In a preferred embodiment, the dark layer absorbs allwavelengths of light across the visible light spectrum, that is from 400nanometers to 700 nanometers wavelength. The dark layer may also containa set or multiple pigment dispersions. Suitable pigments used in thecolor contrast layer may be any colored materials, which are practicallyinsoluble in the medium in which they are incorporated. Suitablepigments include those described in Industrial Organic Pigments:Production, Properties, Applications by W. Herbst and K. Hunger, 1993,Wiley Publishers. These include, but are not limited to, Azo Pigmentssuch as monoazo yellow and orange, diazo, naphthol, naphthol reds, azolakes, benzimidazolone, diazo condensation, metal complex, isoindolinoneand isoindolinic, polycyclic pigments such as phthalocyanine,quinacridone, perylene, perinone, diketopyrrolo-pyrrole, and thioindigo,and anthriquinone pigments such as anthrapyrimidine.

The functional layer may also comprise a dielectric material. Adielectric layer, for purposes of the present invention, is a layer thatis not conductive or blocks the flow of electricity. This dielectricmaterial may include a UV curable, thermoplastic, screen printablematerial, such as Electrodag 25208 dielectric coating from AchesonCorporation. The dielectric material forms a dielectric layer. Thislayer may include openings to define image areas, which are coincidentwith the openings. Since the image is viewed through a transparentsubstrate, the indicia are mirror imaged. The dielectric material mayform an adhesive layer to subsequently bond a second electrode to thelight modulating layer.

The process according to the present invention utilizes a cholestericliquid crystal material comprising a triplet sensitizer and one or morephoto-tunable chiral dopant (PTCD) that changes its chirality uponirradiation of the triplet sensitizer.

A preferred embodiment of the present invention relates to the processof preparing of a reflective film comprising a cholesteric liquidcrystal material with planar orientation, exhibiting a broad waveband ofreflection. Such a film is particularly useful as a broadband reflectivepolarizer for liquid crystal displays.

The cholesteric liquid crystal material used in this preferredembodiment comprises achiral and/or chiral mesogenic compounds, atriplet sensitizer, and one or more photo-tunable chiral dopant thatlose their chirality upon irradiation of triplet sensitizer. Thecholesteric liquid crystal material is coated and aligned onto asubstrate. The cholesteric liquid crystal material is preferablyselected to exhibit a reflection peak in the blue region of visiblelight or in or close to the UV region, but preferably outside thewavelength range used for the irradiation of the triplet sensitizer.Upon irradiation of the triplet sensitizer, the photo-tunable chiraldopant at the top of the layer undergoes a reaction and loses itschirality first, thus reducing the effective concentration of chiralmolecules and thereby the pitch of the cholesteric liquid crystalmaterial, whereas the photo-tunable chiral dopant at the bottom of thelayer will not react as much, resulting in the formation of a pitchgradient and thereby a broadening of the reflection band. The pitchgradient and thereby the bandwidth of the reflection band can becontrolled by careful selection of the amount, type and ratio of thetriplet sensitizer and the photo-tunable chiral dopant. Thus, withinventive method broadband polarizers with a broad reflection band aswell as notch polarizers with a reflection band of limited bandwidth canbe prepared.

In a particularly preferred embodiment, the above effect is furtherenhanced by adding a dye to the cholesteric liquid crystal materialhaving an absorption maximum adjusted to the wavelength of the radiationused for irradiation of the triplet sensitizer. Preferably, a dye isused whose absorption maximum lies outside the reflection wavelengthrange of the reflective film in order to exclude undesired absorptionsduring the use of the film.

The dye produces an intensity gradient of the radiation in the thicknessdirection of the film, thus the difference of the rate and extent oflight absorption by triplet sensitizer which in turn modulatesdegradation of the photo-tunable chiral dopant between the top and thebottom of the film is increased, leading to an increased pitch gradient.

Another preferred embodiment of the present invention relates to thepreparation of a reflective film comprising a polymerized cholestericliquid crystal material with planar orientation, wherein the reflectionwavelength varies in lateral directions along the film, e.g. in form ofa regular pattern. Such a film is particularly suitable e.g. as colorfilter for liquid crystal displays or projection systems, oras—multicolor image for decorative and security uses.

According to this embodiment, a cholesteric liquid crystal material asdescribed for the first preferred embodiment above is coated onto asubstrate and aligned. The cholesteric liquid crystal material ispreferably selected to exhibit a reflection peak in the blue region ofvisible light or in or close to the UV region, but preferably outsidethe wavelength range used for irradiation of the triplet sensitizer. Thealigned cholesteric liquid crystal layer comprising a triplet sensitizerand at least one photo-tunable chiral dopant is then partially exposedto light of moderate intensity, e.g. by covering the layer with aphotomask. In the exposed parts of the layer, the effectiveconcentration of chiral material is reduced, as the photo-tunable chiraldopant decomposes and loses its chirality, therefore the selectivereflection wavelength in this part of the film changes for example togreen. The process is repeated, but with a different shaped photomaskthat covers different parts of the layer. The irradiation dose oftriplet sensitizer is preferably higher than in the first step, whichhas the effect of further decreasing the effective concentration ofchiral molecules in the exposed part of the mixture, so that theselective reflection wavelength in this part changes for example to red.The process can be repeated several times, with different irradiationdoses of the triplet sensitizer, i.e. different intensities and exposuretimes.

It is also possible to use a photomask that comprises different partshaving different transmissivity for the actinic radiation used forirradiation of the triplet sensitizer. For example, it is possible touse as photomask a black and white photocopy or photograph exhibitingdifferent gray shades, which is printed or copied onto a transparentmaterial. Alternatively, it is possible to irradiate selected parts ofthe cholesteric liquid crystal material coating comprising a tripletsensitizer and at least one photo-tunable chiral dopant by means of afinely focused radiation source, such as a laser beam. In this case, thetriplet sensitizer have to be selected to show absorption of theemission wavelength of the laser used for irradiation of the tripletsensitizer.

In another preferred embodiment, the cholesteric liquid crystal materialfurther comprises one or non-photo-tunable chiral dopants which do notshow a substantial change of chirality, but instead retain theirchirality, under the same conditions where the photo-tunable chiraldopant loses its chirality. Thus, the non-photo-tunable chiral dopantsshould retain their chirality when irradiation of the triplet sensitizerresults in complete loss of chirality in photo-tunable chiral dopant. Byadding a selected amount of one or more non-photo-tunable chiral dopantsto the liquids crystal material and an appropriate triplet sensitizer,it is possible to control the position of the reflection band of theresulting coating. Thus, when preparing for example a broadbandreflective polarizer as described in the first preferred embodimentabove or a bistable polymer disperse liquid crystal display sheet withspot color for example a cholesteric liquid crystal material is usedthat comprises one or more non-photo-tunable chiral dopants, the amountand helical twisting power (HTP) of which is selected such that thecholesteric liquid crystal material mixture comprising only thenon-photo-tunable chiral dopants shows reflection of red light. To thismixture are added one or more photo-tunable chiral dopant, the amountand helical twisting power (HTP) of which is selected such that thetotal mixture, comprising both photo-tunable chiral dopant andnon-tunable chiral dopants, shows selective reflection of blue light. Afilm is prepared as described above. The resulting film will have abandwidth extending from a minimum reflection wavelength in the blueregion to a maximum reflection wavelength in the red region. This methodcan be applied analogously to the preparation of patterned films asdescribed above. Thus, the different colors in the different regions ofthe resulting patterned polymer film can be controlled by appropriateselection of the amount and helical twisting power of the photo-tunablechiral dopant and non-tunable chiral dopants.

In a particularly preferred embodiment, the cholesteric liquid crystalmaterial comprises photo-tunable chiral dopant and non-tunable chiraldopants having opposite twist sense. The resulting pitch length of thecholesteric liquid crystal material is then given by the amount andhelical twisting power (HTP) of the two materials with differenthandedness. In this case, triplet sensitized decomposition of thephoto-tunable chiral dopant having a given handedness uponphoto-irradiation will lead to an increase of the effectiveconcentration of the non-tunable material with the opposite handedness,leading to a decrease of the resulting pitch and thus to a decrease ofthe reflection wavelength of the cholesteric liquid crystal material.

The thickness of a reflective film prepared by the inventive method ispreferably from 1 to 30 μm, in particular from 1.5 to 20 μm, verypreferably from 2 to 10 μm. In case of reflective films with a pitchvariation in the direction perpendicular to the plane of the film, thethickness is also influencing the bandwidth of the reflective film.Depending on the band position and the bandwidth, the thickness of thereflective film is preferably from 5 to 30 μm. For bandwidths of about300 nm or more, a thickness of 10 to 20 μm is particularly preferred.For reflective films with smaller bandwidths e.g. in the range from 100to 200 nm a thickness of 1.5 to 10 μm is preferred.

The following examples are provided to illustrate the invention.

EXAMPLE 1

The following example illustrates the effect of a triplet sensitizer S-2(triplet energy E_(T)˜64 kcal/mol) on change in the reflectionwavelength band upon irradiating a chiral nematic liquid crystal mixturecomprising a nematic host, a high twist chiral dopant and aphoto-tunable dopant.

The nematic host mixture BL087 obtained from Merck, Darmstadt, Germanywas combined with a high twist chiral dopant to create a chiral nematicliquid crystal (CLC) mixture with a reflection band centered at 580 nm.The cholesteric liquid crystal mixture was separated into two portions.To one portion (control) was added 5 wt % of PTCD-1,1,1′binapththol(triplet energy, E_(T)˜55 kcal/mol). To the second portion (invention)was added 5 wt % PTCD-1 and 1 wt % S-2. Samples of both the controlmixture (PTCD-1 only) and the invention mixture (PTCD-1 and S-2) wereplaced on a glass slide and subjected to long wavelength UV irradiationat 365 nm. The change in the wavelength of reflection as function of theamount of irradiation was noted.

Sample #1 (Control)

Amount of irradiation (J/cm²) Wavelength of reflection (nm)  0 465 11475 33 485 55 490Sample #2 (Invention)

Amount of irradiation (J/cm²) Wavelength of reflection (nm)  0 465 11485 22 505 44 535 66 550

It is clear that the inventive sample undergoes a much greater change inwavelength upon irradiation of triplet sensitizer S-2 that has tripletenergy higher than the triplet energy of PTCD-1. For example,irradiation of 55 J/cm² changes the wavelength of reflection of thecontrol sample by only 25 nm whereas irradiation of 44 J/cm² changes thewavelength of reflection of the invention sample by 60 mm.

EXAMPLE 2

This example illustrates using the method of the invention to create abi-stable flexible display with spot color.

A cholesteric liquid crystal mixture with reflection band centered at670 nm was prepared by combining the nematic host mixture BL087 with asuitable high twist chiral dopant. PTCD-1 (5 wt %) and S-2 (1 wt %) werethen added to this mixture. The wavelength of reflection was shifted to550 nm.

A dispersion of the above mixture was then prepared as follows. To 241grams of distilled water was added 3.6 grams of Ludox TM-50 colloidalsilica suspension (obtained from DuPont) and a 10 wt % aqueous solutionof a copolymer of methylaminoethanol and adipic acid comprisingequimolar amounts of the two components. To this was added 108 grams ofthe cholesteric liquid crystal mixture. The mixture was stirred using ahigh-speed mixer and homogenized using a Microfluidics M110F homogenizeroperating at a fluid pressure of 3000 psi. The resulting dispersion wasfiltered using a 23 μm filter. The droplet size distribution wasmeasured using a Coulter Counter and the mean droplet size was found tobe close to 10 μm.

The dispersion was combined with an aqueous solution of bovine gelatinto create a coating composition containing 8 wt % liquid crystal and 5wt % gelatin. The composition was spread at a temperature of 45 C. onITO-coated polyethylene terephthalate (PET) obtained from BekaertSpecialty Films, LLC, to give a uniform wet coverage of 61.5 cm³/m². Thesputter coated ITO layer (300 ohm/sq resistivity) had a thickness ofapproximately 240 angstroms. The coating was allowed to dry at roomtemperature (23 C.).

A mask was applied over the dried coating to expose a circular region ofapproximately one-inch diameter. This region was then selectivelyexposed to 366 nm UV radiation to create an area of spot color in thedisplay. After exposure to 12 J/cm² of radiation the wavelength ofreflection in the circular area had shifted from 550 nm to 620 nm andtwo regions of color could be very easily differentiated. A carbon-basedconductive ink (Electrodag 423SS from Acheson Corporation) was thenscreen printed on top of the liquid crystal layer to cover both regions.A voltage pulse (square wave of 1000 Hz and duration of 100 ms) was thenapplied to the sample. It was found that an amplitude greater than 100volts switched both regions into the bright or planar reflecting stateand an amplitude of 40 volts switched the regions into the dark orweakly scattering focal conic state. Both regions were bi-stable.

The invention has been described in detail with particular reference tocertain preferred embodiments thereof, but it will be understood thatvariations and modifications can be effected within the spirit and scopeof the invention.

1. A display comprising a substrate, a liquid crystalline layer thereon,wherein said liquid crystalline layer comprises a nematic host, at leastone chiral dopant, a photo-reacted compound, and a triplet sensitizer,and at least one transparent conductive layer.
 2. The display of claim 1wherein said photo-reacted compound has undergone a photochemicalreaction resulting in the loss of the chirality as a result of tripletsensitization resulting in racemization and/or isomerization of thepreviously photo-reactive chiral form of said photo-reacted compound. 3.The display of claim 2 wherein said photo-reactive chiral form of saidcompound is:


4. The display of claim 1 wherein said photo-reacted compound hasundergone a photochemical reaction resulting in the loss of thechirality as a result of triplet sensitization by decompositionresulting in triplet sensitization bond cleavage reaction of thepreviously photo-reactive chiral form of said photo-reacted compound. 5.The display of claim 4 wherein said bond cleavage reaction affectsphotoremovable protecting groups or photocleavable leaving groups. 6.The display of claim 5 wherein said photoremovable protecting groups areesters of sulfonic acids, carboxylic acids, hydrazones, aryl azidoethers, benzoin esters, or benzyloxycarbonyl compounds.
 7. The displayof claim 5 wherein said photocleavable leaving groups are aryl cyanoacids, aryloxy acetic acids, α-aryl propionic acids, or benzoin esters.8. The display of claim 5 wherein said photocleavable leaving groups arearyl ketones.
 9. The display of claim 1 further comprising at least onenon-photo-tunable chiral dopant.
 10. The display of claim 1 wherein saidtriplet sensitizer comprises thioxanthone.
 11. The display of claim 10wherein said triplet sensitizer is:


12. The display of claim 10 wherein said triplet sensitizer is:


13. The display of claim 1 wherein said triplet sensitizer comprisesketocoumarin.
 14. The display of claim 13 wherein said tripletsensitizer is:


15. The display of claim 13 wherein said triplet sensitizer is:


16. The display of claim 1 wherein said triplet sensitizer absorbs lightabove 400 nm.
 17. The display of claim 1 further comprising at least onecosensitizer.
 18. The display of claim 1 wherein said chiral dopant ispresent in an amount of from 0.1 to 20 weight percent, based on thetotal weight of the liquid crystalline layer.
 19. The display of claim 1wherein said liquid crystalline layer comprises a bistable liquidcrystalline material.
 20. The display of claim 19 wherein said bistableliquid crystalline material comprises a chiral nematic liquidcrystalline material.
 21. The display of claim 20 wherein said liquidcrystalline material further comprises a binder.
 22. The display ofclaim 21 wherein said binder comprises gelatin.
 23. The display of claim19 wherein said liquid crystalline layer comprises a close-packed,ordered monolayer of domains of chiral nematic liquid crystal in a fixedpolymer matrix.
 24. The display of claim 19 wherein said liquidcrystalline layer comprises a pitch gradient.
 25. The display of claim 1wherein said transparent conductive layer comprises ITO.
 26. The displayof claim 1 wherein said transparent conductive layer comprisespolythiophene.
 27. The display of claim 1 further comprising at leastone electrically conductive layer, wherein said liquid crystallineimaging layer is between said transparent conductive layer and said atleast one electrically conductive layer.